Added contributing guide on documentation

CONTRIBUTING.md

@@ -1,3 +1,5 @@
+# Development
+
 1. Install (golang)[https://golang.org/]
 1. Run  `scripts/devtools.sh` to install the required devtools
 2. Run `make [local] vendor` to install the vendor dependencies
@@ -6,3 +8,33 @@
 4. Make the code changes.
 5. `make [local] test` to lint code and run tests
 6. Profit! ok no profit, but raise a PR and get kudos :)
+
+
+# Documentation
+
+The documentation is a bit of a mixed bag (sorry in advanced, I do plan on simplifying it...) - with some parts automatically generated and stiched together and some statically defined.
+
+Documentation is written in markdown, and is published in the 'gitbook' branch.
+
+The various operator documentation (e.g. 'strings') are generated from the 'master' branch, and have a statically defined header (e.g. `pkg/yqlib/doc/operators/headers/add.md`) and the bulk of the docs are generated from the unit tests e.g. `pkg/yqlib/operator_add_test.go`.
+
+The pipeline will run the tests and automatically concatenate the files together, and put them under 
+`pkg/qylib/doc/add.md`. These files are checked in the master branch (and are copied to the gitbook branch as part of the release process).
+
+## How to contribute
+
+The first step is to find if what you want is automatically generated or not - start by looking in the master branch. 
+
+### Updating dynamic documentation from master
+- Search for the documentation you want to update. If you find matches in a `*_test.go` file - update that, as that will automatically update the matching `*.md` file 
+- Assuming you are updating a `*_test.go` file, once updated, run the test to regenerated the docs. E.g. for the 'Add' test generated docs, from the pkg/yqlib folder run:
+`go test -run TestAddOperatorScenarios` which will run that test defined in the `operator_add_test.go` file.
+- Ensure the tests still pass, and check the generated documentation have your update.
+- Note: If the documentation is only in a `headers/*.md` file, then just update that directly
+- Raise a PR to merge the changes into master!
+
+### Updating static documentation from the gitbook branch
+If you haven't found what you want to update in the master branch, then check the gitbook branch directly as there are a few pages in there that are not in master.
+
+- Update the `*.md` files
+- Raise a PR to merge the changes into gitbook.